Currently, there are several types of devices which require a uniform transverse magnetic field for proper operation. For example, magnetic resonance imaging devices require relatively large uniform transverse magnetic fields, that is, they require fields generally above 1 kOe. Presently, superconducting magnets are used to generate these uniform fields. However, superconducting magnet structures are bulky and expensive and, therefore, attempts have been made to replace these superconducting magnet structures with permanent magnet structures. However, during the course of fabricating and assembling these permanent magnet structures, physical and magnetic defects occur. These defects create magnetic field irregularities that are greater than tolerable for the operation of the magnetic resonance images.
Recently, though, a method has been developed which compensates for these magnetic field irregularities wherein magnetic dipoles are positioned within the permanent magnet structure to reduce the deviation from the desired magnetic field to essentially zero. This is fully described in Compensation of Non-uniform Magnetic Properties of Components of a Yokeless Permanent Magnet, IEEE Transactions on Magnetics, Vol. 25, No. 5, pages 3904-3906, September, 1989 by Abele et al and in U.S. application Ser. No. 07/587,285, filed Sept. 24, 1990, entitled, "COMPENSATION FOR MAGNETIC NON-UNIFORMITIES OF PERMANENT MAGNET STRUCTURES", which is incorporated herein by reference. This method employs magnetic dipoles that are placed in symmetrical arrays about the cavity axis of the magnetic structure. Because each magnetic structure typically is composed of sectional slices of permanent magnetic material, the magnetic field of each slice needs to be mapped with Hall probes or other field meters and compared with the desired fields. Then, the dipole strength and orientation for each slice is determined from the measured field deviations as more fully explained in the above noted references. Thereafter, the dipoles are symmetrically placed in each slice as calculated.
Since permanent magnet structures are generally composed of several slices of permanent magnetic material, several dipoles of varying strengths must be employed. Obviously then, custom made magnets would be prohibitive on a mass production basis and therefore, it is desirable to manufacture a single adjustable magnet that can be set to any desired dipole strength and orientation. The present invention offers such a magnet.